Adding ANOTHER Monitor

Hey guys. I'm not sure if this question is up the alley of the forum but it's worth a shot.

I'm trying to connect yet another monitor to my Editing System as a mirror for my secondary monitor. There are several ports on both monitors but none have availed. I have tried to connect via HDMI, VGA, and even S-Video but nothing comes out. Is there any thing I need to change in my Display Settings to make this happen? Or are there any suggestions on what I should try next?

I reckon should resort the manuals but who has time for that. Creative Cow is where real Tech lives and answers get found. Thanks guys for all you do!